The Japan Assessment of Pitavastatin and Atorvastatin in Acute Coronary Syndrome (JAPAN-ACS) trial has found that early aggressive statin therapy in patients with acute coronary syndrome (ACS) significantly reduces the plaque volume (PV) of non-culprit coronary lesions. The purpose of the present study was to evaluate clinical factors that have an impact on plaque regression using statin therapy. Serial intravascular ultrasound observations over 8-12 months were performed in 252 ACS patients receiving pitavastatin or atorvastatin. Linear regression analysis identified the presence of diabetes mellitus (DM) and PV at baseline as inhibiting factors, and serum remnant-like particle-cholesterol level at baseline as a significant factor significantly affecting the degree of plaque regression. Significant correlation between% change of PV and low-density lipoprotein cholesterol (LDL-C) level was found in patients with DM (n= 73, P< 0.05, r= 0.4), whereas there was no significant correlation between the 2 parameters in patients without DM (n= 178). The regression of coronary plaque induced by statin therapy after ACS was weaker in diabetic patients than their counterparts. Moreover, vigorous reduction of the LDL-C levels might induce a greater degree of plaque regression in ACS patients with DM.(Circ J 2010; 74: 1165-1174)